SyncMaster 205BW / Intel i810 timing issued

[Andy Armstrong]

I have a Samsung SyncMaster 205BW attached to a Dell Dimension 3100c
dual booting Ubuntu Feisty and XP Pro. The graphics controller (onboard)
reports itself as

Intel(r)915G/915GV/910GL Graphics Controller

The monitor is connected to the analog VGA port. In XP the display is
fine at 1680 x 1050 - the monitor's native resolution. In Ubuntu it
appears as if there's a horizontal timing problem.

I'm using 915resolution to patch the BIOS to support the monitor's 1680
x 1050 resolution.

The monitor switches to 1680 x 1050 but the display timing looks out -
as if monitor pixels and graphics pixels are slightly differently
spaced. If I display a line of upper case 'I's you can see that the stem
of the letter falls exactly on a monitor pixel boundary only every 35
pixels.

So from that I'm inferring that some aspect of horizontal timing is set
to a value about 34/35 of what it should be :)

Since it works fine under Windows (no setup changes, just reboot) I
decided to find a Windows program that would let me investigate the
display timing. I found PowerStrip[1]. Here are the timings it reports:

http://hexten.net/x.org-205bw-intel/timing.png

They were disappointingly similar to what I already had in xorg.conf -
but I tweaked the dot clock, vertical and horizontal timings to make
them match. Here's the important bit of my xorg.conf:

     Section "Device"
     	Identifier	"Intel 910GL"
     	Driver		"i810"
     	BusID		"PCI:0:2:0"
     EndSection

     Section "Monitor"
     	Identifier	"Samsung SyncMaster 205BW"

         VendorName      "Samsung"
         ModelName       "SyncMaster 205BW"
         DisplaySize     433 270

     	HorizSync       65.510 - 65.510
         VertRefresh     60.156 - 60.156
         Modeline	    "1680x1050:magic"	146.75  \
                     1680 1784 1960 2240 \
                     1050 1053 1059 1089 \
                     +vsync -hsync

     	Option		    "PreferredMode" "1680x1050:magic"
     	Option		    "DPMS"
     	Option		    "NoDDC"
     	Option		    "ModeValidation" "NoMaxPClkCheck"
     EndSection

     Section "Screen"
     	Identifier	"Default Screen"
     	Device		"Intel 910GL"
     	Monitor		"Samsung SyncMaster 205BW"
     	DefaultDepth	24
     	SubSection "Display"
     		ViewPort	0 0
     		Virtual		1680 1050
     		Depth		24
     	EndSubSection
     EndSection

The lines with continuations aren't like that in the real file - I've
just done that for this mail.

The timing in the modeline turns out to be the same as the settings
other people are using with their 205BWs and also what DDC reports if  
you
allow it to.

And it doesn't work. As far as I can see the timings should be exactly
the same as under XP but I'm still getting the 34 screen pixels for 35
monitor pixels thing.

Am I missing something blindingly obvious? Anyone got any ideas?

You can find my complete xorg.conf here:

http://hexten.net/x.org-205bw-intel/xorg.conf.txt

and my Xorg.0.log here:

http://hexten.net/x.org-205bw-intel/Xorg.0.log.txt

[1] http://entechtaiwan.net/util/ps.shtm

-- 
Andy Armstrong, hexten.net